How much better is bigger?

[davidleitch]

Bigger planes (more wing area) fly better than small ones, or so one is led to believe reading here on the 'zone. How much better though? For instance in moving from a 2.5 metre to a 3.2 meter glider what sort of performance improvement should there be, assuming a constant wing loading? Presumably performance is expressed as sink rate in still air? Or alternatively distance covered.

the improvement is due to a better Reynolds number isn't it? I looked up the definition of Reynolds number and there is a length parameter in the numberator, but that is the extent of my knowledge.

[davidleitch]

Pierre Audette for instance noted in a Re thread

Since models run at lower Re than their full size counterparts, they are not as efficient in creating lift, and generate more drag.

However this doesn't give me a sense of how much better bigger is. Nor why the smaller model is worse, is it the parasitic or the induced drag that's the problem.

Its a practical problem as I'm trying to work out whether the advantage of a 3 or 3.2 metre glider over a 2.5 meter one will justify the effort and cost .

[Ollie]

The reduction in sinking speed due to increased aspect ratio where all the other things are equal will be greater than the increased reynolds number effect where everything including the aspect ratio is kept the same for the two cases.

You can verify this by running PC Soar for the two cases. See:
http://my.athenet.net/~atkron95/pcsoar.htm

Your question rules out wing loading reduction as an approach to reducing sinking speed. It is, of course, the most effective way to decrease sinking speed. It is usually accompanied by a reduction in wind penetration ability. However, by using low wing loading in conjunction with thin, low camber airfoils, efficient lift distribution wing design and low parasitic drag the wind penetration ability can be largely retained. Dr. Drela's designs are cases in point. There is a double pay off to low wing loading. It not only lowers the sinking speed in straight line flight but it allows smaller circles for the same sinking speed in turns. This improves the ability to core thermals, particularly small, low altitude thermals.

[Sail 'n Soar]

You have given little information other than you are considering gliders of 2.5 metre and 3 - 3.2 metre spans with identical wing loadings.

Try the site, below, put in your parameters and see the relative benefits for yourself. This will give you the plots of CL vs. CD for your airfoil choice, model area, span and weight. However, it does not include induced drag or the drag coefficient contribution from the fuselage and stab.

http://soaring.cnde.iastate.edu/calcs/frames.shtml

Can I assume you are considering gliders with similar aspect ratios? Unless you are dealing with very high aspect ratio gliders, the answer is that the 3.2 metre glider will be a little bit better. Nevertheless, the 2.5 metre gliders are a very nice size, fly very well, are more maneuverable/turn tighter with faster roll rates, are less expensive and are easier to transport and store.

I guess you know which way I would go!

[davidleitch]

Well the 2.5 metre is an Organic and the particular 3.2 meter is an FVK K2 light.

However its my perception that just about anyone who flys F3F or F3J and 95% of the F3B pilots will be using a 3 metre or larger glider.

Again its descriptive analysis to suggest that they must see some advantage in using larger wingspans, and that has to be more than visibility.

I tried the program however the plot of minimum sink wasn't working either for the K2 data or for the default data.

Its interesting to note that aspect ratio is an important determinant of minimum sink.

[Ollie]

There are two kinds of wing loading. The most commonly used is wing area loading. The less commonly used wing loading is wing span loading. The lighter the span load the lower the sinking speed up to the point where profile drag increase, at very, very low reynolds numbers, is no longer off set by induced drag savings due to increased aspect ratio. Where the two drag considerations cross over is dependent on the size of the model, the airfoil and the wing area loading. Each particular combination has to be examined to find that crossover point when looking for a design optimized for minimum sinking speed or maximum glide angle. A progran like PC Soar is the easiest way to get the answer if you can get it to work. If you can get the L/D display to work it will also serve to find minimum sinking speed. Just draw a line from the origin tangent to the plane's L/D vs. airspeed polar. That line corresponds to the minimum sinking speed. Airspeed divided by sinking speed equals L/D.

[Sail 'n Soar]

Originally posted by davidleitch
Well the 2.5 metre is an Organic and the particular 3.2 meter is an FVK K2 light.

However its my perception that just about anyone who flys F3F or F3J and 95% of the F3B pilots will be using a 3 metre or larger glider.

Again its descriptive analysis to suggest that they must see some advantage in using larger wingspans, and that has to be more than visibility.

I tried the program however the plot of minimum sink wasn't working either for the K2 data or for the default data.

Its interesting to note that aspect ratio is an important determinant of minimum sink.

OK, so you are thinking of competition. In that case, go for the larger spans. The small differences in performance can result in big differences in contest placement. The larger wing spans result in either higher aspect ratios, with lower induced drag, or larger cords at a given aspect ratio, with lower profile drag due to the higher Reynolds numbers. In either case the the larger span glider will have a higher L/D, shallower glide slope, and a lower sink rate for a given wing loading and with everything else equal.

The two models you mentioned have almost identical 14.7:1 aspect ratios, so the coefficient of induce drag (CDi) for a given CL will be basically equal. Both have relatively large cords for a model, so you aren't worried about very low Reynolds numbers problems. That leaves you with a slight improvement in CD due to the ~25% higher Reynolds numbers at a given speed and a slightly reduced profile drag due to the slightly smaller ratio of the fuselage area to the wing area. i.e., bigger is better.

However, if you are a sport flyer, I recommend you weight higher the other benifits of the smaller span gliders when you make your decision. The amount of time you can stay up in thermalling conditions will be dictated more by your skill than by your choice of either of the two mentioned models.

[davidleitch]

Sail n Soar

Thank you for a precise comment.

I do appreciate the benefits of the Organic, I am very fond of it. However I am also ambitious to try larger models. The inconvenience of larger models does weigh on my mind.

I am very well aware that skill is the major factor. Its one reason, besides pleasure and relaxation, that I try to fly something every day. I played guitar for 20 years and equipment makes a difference but skill is vastly more important.

cheers
Dave